1. Field of the Invention
This invention relates to a semiconductor memory device, EEPROM, which is formed with electrically rewritable and non-volatile memory cells arranged.
2. Description of Related Art
EEPROM flash memories are roughly classified into two types, i.e., NAND-type and NOR-type. A NAND-type flash memory uses a NAND string (i.e., NAND cell unit) with plural cells connected in series as sharing sources and drains with the neighboring cells. Due to this fact, the NAND-type flash memory has a higher cell density than a NOR-type flash memory. In addition, since NAND-type flash memory is able to be written into plural cells at a time by FN tunneling, power consumption thereof is small. In consideration of these characteristics, the NAND-type flash memory is generally applied to a file memory with a large scale capacitance.
On the other hand, NOR-type flash memory is characterized in that it is high-speed accessible, while the power consumption is large because hot electron injection is used for data writing. Therefore, the NOR-type flash memory is mainly applied to mobile devices.
Recent mobile devices are going to treat image data and the like with large data quantities. Accordingly, it is required of a flash memory to have high-speed accessibility and a large capacitance like a file memory. As described above, a conventional NAND-type flash memory, cell current of which is small due to its NAND string structure, is not suitable for high-speed random accessing. To apply a NAND-type flash memory to a high-speed system with a buffer memory such as a DRAM or the like, such a method is used as, for example, to read out one page data to a page buffer, then serially transfer and output the read out data, thereby improving a data transmission rate.
Even if the above-described method is used, the performance improvement of the conventional NAND flash memory is limited. The reason of this can be said as follows. It is impossible to use a reference level for high-speed sensing because the cell current of the NAND-type flash memory is about 1/100 to 1/10 of that of the NOR-type flash memory. The sense amplifier used in the conventional NAND flash memory is configured to sense cell data by sensing whether the data charge in the latch is discharged or not in response to the on/off state of a selected cell. Therefore, it takes about a few or several micro seconds for data reading. In contrast to this, in the NOR-type flash memory, the cell dada read may be done in 10 to 100 [nsec].
One approach for increasing the cell current of the NAND-type flash memory is to make the cell size (i.e., channel width) large. However, this approach will dilute a property of the NAND-type flash memory as the unit cell area is small.
It has already been provided to use a multi-value data storage scheme in order to make a flash memory able to store a large data quantity. It has also been provided a method for shortening read time by reducing the number of data read steps in a case that multi-value storage scheme is used (see, e.g., Unexamined Japanese Patent Application Publication No. 2001-93288).
Further, it has already been provided a memory device, in which two memory cells coupled to a bit line pair and simultaneously selected constitute a pair cell, and the pair cell stores multi-value data defined by a combination of different threshold voltages (for example, Unexamined Japanese Patent Application Publication No. 2004-319007).